Kinetic analysis of Li|Li+ interphase in an ionic liquid electrolyte

Kinetic analysis of the Li|Li+ interphase in an electrolyte based on N-metyl-N-propylpyrrolidinium bis(trifluoromethanesulfon)imide ionic liquid (MPPyrrTFSI) and lithium bis(trifluoromethanesulfon)imide salt (LiTFSI) was performed. Li|electrolyte|Li and LiC6|electrolyte|Li cells were galvanostatically charged/discharged in order to form solid electrolyte interphase (SEI) protecting layer. SEM images showed that the surface of both Li and LiC6 anodes was covered with small particles. The fitting procedure of electrochemical impedance data taken at different temperatures gave three resistances (Rel, RSEI, Rct) and hence, three lnR = f(T−1) straight lines of different slopes. Specific conductivity and activation energy of the conduction process of the liquid electrolyte, were ca. σ = 2.5 mS cm−1 (at T = 25.0 °C) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ E_{\text{el}}^{\# } $$\end{document} = 15 kJ mol−1. Activation energy for the conduction process in the SEI layer was ca. 56 kJ mol−1 in the case of the metallic lithium and 62 kJ mol−1 for the graphite anode. Activation energy of the charge transfer process, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ E_{\text{ct}}^{\# } $$\end{document}, for Li and LiC6 anodes was 71 and 65 kJ mol−1, respectively. Analysis of literature data for different electrolytes suggests that the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ E_{\text{ct}}^{\# } $$\end{document} value for Li+ reduction may be approximated by 57 ± 5 kJ mol−1. Activation energy for the diffusion processes in the graphite electrode, detected from the Warburg coefficient, was ca 74 kJ mol−1.